Optical system operating with variable angle of indidence

ABSTRACT

An optical system for use in measurements in a sample comprising a light source ( 102 ) operable to produce an incident light beam propagating in a certain direction towards the sample (S) through an illumination channel (IC), a detector unit ( 104 ) for collecting light coming from the sample through a detection channel (DC), and generating data indicative of the collected light, a light directing assembly ( 106 ) operable to direct the incident beam onto a certain location on the sample&#39;s plane with a plurality of incident angles, and to direct light returned from the illuminated location to the detector unit ( 104 ), the light directing assembly ( 106 ) comprising a plurality of beam deflector elements ( 108  A-D), at least one of the deflector elements being movable and position of said at least one movable deflector element defining one of the selected incident angles.

FIELD OF THE INVENTION

The present invention is generally in the field of opticalmonitoring/inspection techniques, and relates to a spectrometer methodand system.

BACKGROUND OF THE INVENTION

Spectrometer-based techniques, such as spectrophotometry and spectralellipsometry, are widely used in microelectronics for measuring thinfilm properties and line profiles. An object to be measured may be asite on a semiconductor wafer (multi-layer stack) that may have withuniform thin-film structure, or may be patterned, e.g., line array in atleast one layer of the multi-layer stack. If wavelength of incidentlight is of the order of the line array period, reflected light iszero-order diffracted light, while higher orders are scattered indifferent directions and do not reach an optical detector oriented to becapable of detecting the reflected light (scatterometry). For both thespecular reflection and zero-order diffraction, the light used formeasurements may be polarized. Measurement schemes may be of the kindbased on measuring intensity of light (spectrophotometry) orpolarization changes (ellipsometry).

An object under measurements, either of uniform or patterned structuremay thus be described by an optical model with a set of parameters, suchas optical constants, thickness of each layer, pattern geometry andprofile. Measurement of a single spectrum is usually incapable ofdetermining more than two-three unknown parameters of this set with highaccuracy and high level of confidence. Hence, if more parameters of theoptical model of a measured structure are unknown and are to besimultaneously determined from spectral measurements, more suchmeasurements are to be independently carried out. The known approachesto increase the number of independent measurements (applicable for boththe spectrophotometry and ellipsometry) are generally based on thefollowing:

1. Step-by-step removal and measurements of the same stack downward tothe substrate layer. Although this approach enables as many independentmeasurements as needed, it is destructive and is hard to implement formeasuring wafers in production.

2. Step-by-step deposition and measurements of the stack layers. Thisapproach provides a required number of measurements, but is timelyconsuming.

3. Individual deposition of each or some critical layers on a substrateand separate measurements of each of these layers. By this, measurementsin a simple single-layer structure allows for determining both theoptical properties and thickness of the single layer. The single-layermeasured parameters can then be excluded from unknown parameters of thewhole stack. This technique is also time consuming, requires a largenumber of test wafers, and does not takes into consideration the factthat the optical properties of a specific layer may be different whenthe layer is a part of a multi-layer stack, and therefore does notprovide confident information.

4. Applying measurements to the same wafer in different ambiences, e.g.,in air and in water, etc. This technique provides more information aboutthe wafer, but a different ambience might cause some chemical changes ofthe wafer's top layer, such as photoresist.

5. Measurements of the same site on a wafer with different angles oflight incidence. FIG. 1 schematically illustrates such a variable anglemeasurement system 10, for example the H-VASE model commerciallyavailable from J.A. Woolam Co., Inc. The system has two arms 12 and 14mounted for pivotal movement about a pivot axis 16 along a curved frame18. The arms 12 and 14 are L-shaped and carry a light source 20 and adetector 22, respectively. In order to cover the entire surface of awafer W, the latter is supported on a movable X-Y stage 24, whichpreferably is also Z-adjustable. This approach provides more informationthan the single incident angle technique, but suffers from the followingdrawbacks:

-   -   long measurement time, which while being acceptable for material        characterization, is not acceptable for production;    -   a measurement system of a large footprint and size, which does        not complies with the requirements for a measurement system in        production, e.g., integrated metrology system;    -   requirement for a massive and rigid mechanical platform, in        order to provide accurate changing of the angle of incidence        that can hardly be used for integrated metrology.

SUMMARY OF THE INVENTION

There is accordingly a need in the art to facilitate opticalmeasurements in a sample, especially a multi-layer structure, byproviding a novel method and system operable with variable angles ofillumination and light collection.

The present invention provides for varying the angle of incidence oflight onto a specific location (site) on the sample and, optionally alsofor varying the angle of collection of light returned from this site.The system according to the invention requires neither movement of alight source nor of a light detector, but rather utilizes a lightdirecting assembly having a plurality of deflector elements defining aplurality of incident angles. Interaction of an incident beam with aselected one of the deflector elements provides a selected angle ofincidence of the beam onto the sample. Preferably, the plurality ofdeflector elements has deflector elements located in a detection channelas well, thereby enabling interaction of the returned beam with aselected deflector element and accordingly a selected angle of lightcollection by a detector unit.

There is thus provided according to one aspect of the present invention,an optical system for use in measurements in a sample, the systemcomprising:

(a) a light source operable to produce an incident light beampropagating in a certain direction towards the sample through anillumination channel;

(b) a detector unit for collecting light coming from the sample througha detection channel, and generating data indicative of the collectedlight;

(c) a light directing assembly operable to direct the incident beam ontoa certain location on the sample's plane with a plurality of incidentangles, and to direct light returned from the illuminated location tothe detector unit, the light directing assembly comprising a pluralityof beam deflector elements, at least one of the deflector elements beingmovable, a position of said at least one movable deflector elementdefining a selected one of the incident angles.

According to one embodiment of the invention, the plurality of thedeflector elements has two arrays of the deflector elements, one arraybeing located in the illumination channel and the other array beinglocated in the detection channel. Each of the arrays may be formed bydeflector elements arranged in a spaced-apart relationship along therespective channel, in which case the deflector elements may be planarmirrors with or without associated focusing lenses, at least one of thedeflector elements being movable. Each of the arrays may be formed by areflecting surface of a parabolic-sector mirror, in which case the lightdirecting assembly preferably comprises at least one movable planarmirror in the illumination channel, and preferably also comprises amovable planar mirror in the detection channel. According to anotherembodiment of the invention, the plurality of deflector elementscomprises a single parabolic-sector mirror that faces the sample's planewith its reflecting surface. In this case, the light directing assemblycomprises planar mirrors, at least one being movable between a pluralityof operative positions thereby defining the plurality of the incidentangles.

According to another aspect of the present invention, there is provided,a method for measuring in a sample, the method comprising:

(i) providing an incident light beam propagating in a certain directiontowards the sample along an illumination channel;

(ii) directing the incident beam onto a certain location on the sample'splane with a plurality of incident angles, said directing comprisingdeflecting the incident beam by a selected one of a plurality ofdeflector elements resulting in the selected one of the angles ofincidence of the beam onto said certain location.

BRIEF DESCRIPTION OF THE DRAWINGS

In order to understand the invention and to see how it may be carriedout in practice, a preferred embodiment will now be described, by way ofnon-limiting example only, with reference to the accompanying drawings,in which:

FIG. 1 is a schematic illustration of the prior art multi-angle opticalsystem;

FIG. 2 is a schematic illustration of an optical system according to oneembodiment of the invention;

FIGS. 3A to 3D more specifically illustrate three possible examples,respectively, of a deflection element suitable to be used in the opticalsystem of FIG. 2;

FIG. 4 schematically illustrates an optical system according to anotherembodiment of the invention;

FIG. 5 is a schematic illustration of an optical system according to yetanother embodiment of the invention, utilizing the multi-angleillumination/detection and also normal-incidence illumination/detectionand/or imaging;

FIGS. 6A and 6B illustrate two more examples of using normal-incidenceillumination/detection or imaging channel in the optical system of theinvention;

FIGS. 7A and 7B illustrate optical systems according to yet anotherexamples of the invention designed for carrying out multi-angle andnormal incidence measurement/monitoring, as well as imaging andauto-focusing functions;

FIG. 8 is a schematic illustration of yet another embodiment of theinvention, wherein a deflector assembly comprises a parabolic-sectormirror; and

FIG. 9 illustrates how the system of the present invention can be usedfor measuring diffraction efficiency for the first (negative)diffraction order additionally to zero order standard measurements.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 shows the prior art optical system capable of providing variousangles of incidence of a light beam onto a sample.

Referring to FIG. 2, there is illustrated an optical system 100according to one embodiment of the invention applied to a sample (e.g.,wafer) S. The system 100 comprises such main constructional parts as alight source 102 (e.g., a Xe lamp) producing an incident light beam B,which in the present example propagates from the light source parallelto an axis OA normal to the sample's plane; a detector unit 104 (e.g.,spectrophotometer); and a light directing assembly, generally at 106,comprising a plurality of deflector elements. Light beam B propagatesthrough an illumination channel IC, impinges onto the sample at alocation L, and returned light B′ (reflected and/or scattered)propagates through a detection channel DC towards the detector 104. Thesample is typically supported on a stage, and in order tomeasure/monitor a plurality of sample's sites a relative displacementbetween the sample and the optical system (at least deflector assembly)is provided, preferably by moving the stage with respect to the opticalsystem. The stage may be of different types, i.e., x,y- or r,θ-movablestage for scanning the entire surface of the wafer, and z-movable forbringing the measurement site to the best focus of the optical system.

In the present example of FIG. 2, the plurality of deflector elements inthe light directing assembly 106 has two arrays of deflector elements108 and 110 located at both sides of the axis OA. The array 108 isassociated with the light source 102 (illumination channel IC) and thearray 110 is associated with the detector unit 104 (detection channelDC). Additionally, in the present example of FIG. 2, each of the arrays108 and 110 has separate (discrete) deflector elements—four suchelements 108A-108D and 110A-110D in the arrays 108 and 110,respectively, which are arranged in a spaced-apart relationship withinthe respective channel. All the deflector elements, except for thelowermost ones (closest to the sample's plane) 108D and 110D, aremovable to be in and out of the optical path of the respective beam(incident or returned), e.g., are mounted for a reciprocating movementalong X- or Y-axis. Generally speaking, each beam deflector element inthe array can be in its operative state, in which it is in the path ofthe respective light beam, and can be in its inoperative state, in whichit is out of the path of the respective beam. This can be implemented indifferent ways, namely, by reciprocating, rotating or pivotal movementof the element with respect to the path of the light beam propagationtowards the element. When the selected one of the deflector elements isin the optical path of the light beam, all other elements are out ofthis path. For example, the deflector elements can be sequentiallyshifted between the two positions. This can be realized by locating theuppermost deflector element 108A in the path of the emitted beam B,thereby automatically preventing the beam interaction with the otherelements 108B-108D, and providing the beam incidence onto the location Lon the sample with a certain incident angle θ₁. In this case, thedeflector elements 110A-110D of the detection channel are arranged suchthat the deflector element 110A only is in the path of the specularlyreflected beam B′. Then, the uppermost element 108A is shifted away fromthe axis OA, thereby automatically providing the incident beamdeflection by the next element 108B, etc. It should be understood thatin order to provide the operative state of a selected deflector element,not necessarily this selected element is to be moved, but rather one ormore other elements of the deflector assembly, as will be exemplifiedfurther below.

As shown in FIG. 2, the incident beam interaction with a different oneof the deflector elements 108A-108D results in a different angle ofincidence of the beam B onto the same location (point) L on the sample.To this end, the deflector elements are differently oriented withrespect to the axis OA. In the example of FIG. 2, variation of the angleof light collection (detection) is aimed at collecting only specularreflections of the incident beam. It should also be noted that themovable elements of the illumination and detection channels forming apair with respect to the certain incident/collected angle, e.g.,elements 108A and 110A, can be associated with a common drive.

The number of different angles of incidence/collection depends on theopto-mechanical design of the system 100. Practically, the provision offour-five angles is sufficient for most of the inspection/monitoringapplications, enabling the system (including both the illumination andthe detection channels) to be compact, up to the wafer size (200-300mm).

Since each deflector element in the arrays of the illumination anddetection channels can be separately aligned, a very high accuracy ofalignment can be achieved, which enables to provide accurate location ofthe illumination spot (location L) on the chosen measurement site.During the alignment procedure, a nominal angle of incidence may bechanged within a certain tolerance. However, accurate measurementsrequire knowledge of each angle of incidence with high accuracy. Thisproblem can be solved by using a calibration stage. This can for examplebe implemented in the following way. A known object is measured, e.g.,SiO₂ layer on Si with the known thicknesses (commercially available fromVLSI Standards Inc), and the actual angle of incidence is calculated byreaching the best fit between measured value of the film thickness incalibration standard parameters and its specified value.

It is typically desirable to set a measurement spot on the same place(x, y, z) on the wafer for all the angles of incidence, which ispractically difficult to implement with the currently availablealignment technique. To solve this problem, the coordinates (x, y, z) ofthe measurement spot may be determined for each incident angleseparately by carrying out another calibration procedure. A calibrationtarget in the form of a small mirror, e.g., chrome-on-glass mirror of a50 μm diameter may be used. By changing x, y and z coordinates of thewafer supporting stage, a maximal signal of the returned light(reflection) is determined for each incident angle. This correction ofcoordinates (Δx, Δy and Δz) is stored in the memory of the measurementsystem (a control unit). Thus, while changing the angle ofincidence/collection by selecting a corresponding pair of the deflectorelements, the position of the wafer stage is changed accordingly tocompensate for Δx, Δy and Δz for this specific angle.

Reference is now made to FIGS. 3A-3D showing several examples of theconfiguration of the separate deflector elements suitable to be used inthe system 100. In the example of FIG. 3A, the deflector element 208 hasa planar reflecting surface (mirror), which may be associated with alens 209 for beam focusing purposes, as shown in FIG. 3B. In the exampleof FIG. 3C, a deflector element 308 has a curved (concave) reflectingsurface (preferably, a parabolic mirror) thereby providing both thedeflection and focusing of the incident beam. As shown in FIG. 3D, adeflector assembly part 408 (associated with either illumination ordetection channel) can be formed with discrete deflector elements408A-408D supported in a step-like structure, so as to be moved togetherby a common drive (not shown). It should be noted that the deflectorelements used in arrays of the illumination and detection channels arenot necessarily of the same type, but may be of different types. Forexample, the illumination channel may include the curved mirrors, whilethe detection channel may include planar mirrors.

FIG. 4 illustrates an optical system 500 according to another embodimentof the invention. To facilitate understanding, the same referencenumbers are used for identifying components that are common in systems100 and 500. In the system 500, the plurality of deflector elements in alight directing assembly 506 includes two symmetrical sectors 508 and510 of a parabolic mirror accommodated in, respectively, illuminationand detection channels and oriented such that the measurement site(location L) is located in the focal point of the mirror. Furtherprovided in the light directing assembly 506 is a pair of movable planarmirrors 512 and 514 located in, respectively, illumination and detectionchannels IC and DC. In the present example, in distinction to thepreviously described ones, each of the continuous parabolic reflectingsurfaces of the mirror sectors 508 and 510 presents an array ofdeflector elements (deflecting locations), providing a plurality ofdifferent angles of incidence/collection. In order to select a desiredone of the incident angles, the mirror 512 is moved along the X-axisinto a corresponding position with respect to the mirror sector 508.Similarly, in order to collect light returned from the sample with adesired angle (e.g., specular reflection), the mirror 514 is brought toa corresponding position along the X-axis. It should be understood thatthe provision of a continuous parabolic surface allows for obtaining thelight response intensity (e.g., specular reflection) as a continuousfunction of angle of incidence.

In the systems 100 (FIG. 2) and 500 (FIG. 4), multiple, non-zero anglesof incidence of the beam onto the sample are utilized. FIG. 5schematically illustrates how such a system can be easily modified inorder to provide also measurement/monitoring of the sample with thenormal incidence (zero-angle). A system 600 is shown having a lightsource 602, a detector unit 604, and a light directing assembly 606comprising the plurality of deflector elements (including two arrays ofdeflector elements 608 and 610 in the present example), mirrors 612, 614and 616, and a beam splitter 618. The detector unit 604 has aspectroscopic detector 605 and a tube lens 622. Optionally provided inthe system 600 are polarizing elements 620 and 621 located,respectively, in the optical path of a light beam emitted by the lightsource 602 and a light beam propagating to the detector unit. Mirrors612 and 616 are mounted for movement (e.g., rotation) between theiroperative and inoperative states.

When the mirrors 612 and 616 are in the operative state, i.e., in thepath of the incident and reflected beams B and B′, respectively, themulti-angle mode is realized. The beam B is reflected by the mirror 612to propagate through the multi-angle illumination IC, thereby resultingin the beam incidence onto the sample with a certain non-zero angle ofincidence, and the returned beam B′ propagates through the multi-angledetection channel DC to be reflected by the mirror 616 to the detectorunit 604. By shifting the mirrors 612 and 616 into their inoperativestate, i.e., being out of the optical path of the beams B and B′, thebeam B is directed towards the beam splitter 618 to propagate through anormal illumination/detection channel IC′. The beam splitter 618reflects the beam B to propagate to the sample along the axis OA throughan objective lens OL. A specular reflection B′_(nor) of the normalincident beam B_(nor) propagates along the axis OA through the beamsplitter 618, and is reflected by the mirror 614 to the detector unit604.

It should be understood that the normal incidence illumination/detectionchannel can utilize separate illuminator and detector, other than thoseused in the multi-angle illumination/detection system. This isschematically shown in FIGS. 6A and 6B. In the example of FIG. 6A, anoptical system 700A has a multi-angle illumination/detection sub-systemconstructed similarly to the system 100 of FIG. 2, and a normalillumination/detection sub-system 701. The sub-system 701 has a lightsource 703, a detector 704, and a suitable optics including a beamsplitter BS, an objective lens OL, and a tube lens TL, and isaccommodated such that, in each relative location of the sample relativeto the optical system 700A, the same site L is measured/monitored byboth sub-systems 100 and 701A. An optical system 700B of FIG. 6B,similar to the system 700A, utilizes the sub-systems 100 and 701, butwith the sub-system 701 being applied to a measurement site L₁spaced-apart from the site L to which the sub-system 100 is applied. Inthis case, a relative distance between the sites L and L₁ is determinedin a simple calibration procedure and stored in the system memory.

It should be understood that the normal incidence/detection sub-system701 can be used as an imaging module, being a microscope with a CCDdetector and suitable illumination and imaging optics. A control unit(not shown) of the system thus has an electronic card (frame grabber)for grabbing a video signal from the CCD, and an image processingsoftware (including pattern recognition) for processing the grabbedsignal. The pattern recognition software allows for identifying themeasurement site and, in combined operation with X, Y, Z movable system(stage), allows for localizing the required measurement site in themeasurement position (site L) as a point of intersection between theaxes of the illumination and detection channels.

Reference is made to FIGS. 7A and 7B illustrating optical systems 800Aand 800B, respectively, each designed for carrying out multi-angle andnormal incidence measurement/monitoring, as well as imaging andauto-focusing functions. To facilitate understanding, the same referencenumbers are used for identifying the similar components in the systems800A and 800B. The systems differ from each other in that thepluralities of deflector elements of the systems 800A and 800B comprise,respectively, arrays of separate spaced-apart deflector elements (808Aand 810A) and two continuous deflecting surfaces defining the arrays ofdeflector elements (808B and 810B).

Thus, the system 800A (and 800B) comprises a multi-angle andnormal-incidence measurement sub-system 801 utilizing a light source 802(e.g., a Xe lamp with appropriate optics) and a detector unit 804 (e.g.spectrophotometer based on grating and photo-diode array); and animaging and auto-focusing sub-system 805 utilizing a light source 806(QTH lamp with appropriate optics) and a detector unit 807 (including aCCD camera). The sub-system 801 further comprises a light directingassembly 809 including the plurality of deflector elements, mirrors 812A(or 812B in system 800B), 814 and 816A (or 816B in system 800B), a beamsplitter 818, and objective and tube lenses 819 and 820. Preferably, thelight directing assembly of the sub-system 801 also comprises polarizingelements 821 and 822. The detector unit 804 preferably also comprises anaperture stop 823 and a pinhole aperture 824. The aperture stop 823 isused for adjusting numerical aperture of the reflected light beam. Thepinhole aperture 824 is used to limit a measured area on the wafer W.

In both systems 800A and 800B, each of the mirrors 812A (or 812B) and816A (or 816B) is shiftable between its inoperative state (out of theoptical path of the respective beam) and operative state (in the path ofthe respective beam) as described above with reference to FIG. 5. In thesystem 800A, at least some of the deflector elements in the arrays 808Aand 810A are movable to provide operative and inoperative state of theselected deflector element in each array, while in the system 800B, theparabolic mirrors 808B and 810B are stationary mounted, and mirrors 812Band 816B are additionally movable along the X-axis between differentoperative positions thereof to thereby provide the operative state ofthe selected deflector element in the arrays (deflecting location).

When the mirrors 812A and 816A are in the operative state, a beam B fromthe light source 802 is directed to the sample through the multi-angleillumination channel IC (with only one deflector element in the array808A being currently operative), and a reflection B′ of the inclinedincident beam B propagates through the multi-angle detection channel DCto the mirror 816B, which reflects the beam B′ to the detector unit 804.When the mirrors 812A and 816B are inoperative, the beam B is reflectedfrom the beam splitter 818 to normally impinge onto the same location Lon the sample, and a reflection B′_(nor) passes through the beamsplitter 818, and is reflected by the mirror 814 to the detector 804.

The sub-assembly of the light directing assembly including the beamsplitter 818 and mirror 814 is also movable (e.g., along the X- orY-axis) between its operative and inoperative position to be,respectively, in and output of the optical path of the respective beams.When this sub-assembly is operative, the system 800A functions asdescribed above, namely, carries out measurements with the sub-system801. In order to operate the sub-system 805 to carry out patternrecognition and/or auto-focusing, the sub-assembly 818-814 is shiftedinto its inoperative state.

The sub-system 805 comprises a grid assembly 825, a beam splitter 826for spatially separating incident and reflected beams B_(inc) andB_(ref), and a tube lens 827. There are several ways to reach the bestfocus. As shown in the figure, beam B_(inc) passes through the gridassembly 825, is reflected by the beam splitter 826, and is focused ontothe sample by the tube lens 827 and the objective lens 819. Thereflected beam B_(ref) propagates back to the CCD 804 through the beamsplitter 826. The use of the grid assembly 825 is aimed at finding thebest focus by analyzing the image of line arrays (grid) projected on thesample plane through the illumination channel. This technique isdisclosed in U.S. Pat. No. 5,604,344 assigned to the assignee of thepresent application. Generally, another auto-focusing sensors may beapplied as well.

Turning now to FIG. 8, there is illustrated yet another embodiment of anoptical system 900 according to the invention. The system 900 comprisesa light source 902 optionally associated with a polarizing element 903;a detector unit 904 (e.g., including a spectrophotometer) alsooptionally associated with a polarizing element 905; and a lightdirecting assembly 906. The light directing assembly 906 comprises adouble-side movable planar mirror 908, a wavelength-selective beamcombiner 910 (e.g., transparent for wavelength more than 750 nm andreflective for wavelengths up to 750 nm), three planar mirrors 912, 914and 916, and a parabolic-sector mirror 918 whose continuous reflectingsurface facing the sample's plane presents a plurality of deflectorelements. The mirror 918 is formed with an opening 918A (generally, anoptical window), where for example an objective lens 919 can be located.The provision of the optical window enables a normal incidenceillumination/detection. The wavelength selectivity of the mirror 910 isoptional and is associated with the provision of an imaging channelformed by an additional light source 920, an imaging detector unit 922(e.g., CCD camera), a beam splitter 924 and a tube lens 925. It shouldbe noted that the use of the additional light source 920 is optional,and the same light source 902 can be used in the imaging channel, inwhich case the mirror 908 is shifted into its extreme left positiondenoted 908′. Further provided in the light directing assembly is a beamsplitter 926 serving for the normal incidence measurement, as well asfor imaging purposes in this specific configuration. A grid assembly 927is appropriately accommodated in front of the light source 920 forauto-focusing purposes.

It should be noted that the double-side movable planar mirror 908 can bereplaced by two single-side planar movable mirrors. The dimensions ofthe mirror 908 (thickness and length) are defined by the predeterminedplurality of incident angles, as well as by the configuration of theoptical scheme, including dimensions of the beam splitter 926, etc.

The system 900 operates in the following manner. In the multi-angleillumination/detection mode, mirrors 908, 910, 912, 914 serve forsequential reflection of a light beam B emitted by the light source 902towards the mirror 918. The position of the mirror 908 along the X-axisdefines the deflector element (deflecting location) on the continuousreflecting surface of the mirror 918 on which the light beam B wouldimpinge, and therefore defines the angle of incidence of the beam B ontothe location L on the sample S. In the normal-incidence mode, the mirror908 is in its extreme position 908′, and the beam B is sequentiallyreflected by mirrors 908′ and 910, and is then reflected by the beamsplitter 926 to propagate through the opening 918A to the location L onthe sample. The normally reflected beam would propagate back through thebeam splitter 926 towards the detector unit 904. In the imaging mode,beam B″ produced by the light source 920 is reflected from the beamsplitter 924 and transmitted by the wavelength-selective mirror 910 tobe further reflected from the beam splitter 926 towards the location Lalong the axis perpendicular to the sample's plane. A normally reflectedimaging beam propagates back and is reflected by the beam splitter 926to the CCD 922.

As indicated above, an imaging channel may be combined with a normalincidence illumination/detection channel. This allows for combiningspectral measurements at both normal and oblique illumination in thesingle measurement system. In the case of spectrometry, this alsoallows, in some cases, for measuring diffraction efficiency for thefirst (negative) diffraction order additionally to zero order standardmeasurements. A schematic beam propagation scheme is schematicallyillustrated in FIG. 9. The patterned sample S has a grid G (line array).An illuminating beam B impinges onto the grid with a certain non-zeroangle of incidence, and the first negative diffraction order B₁, angle(−φ), and a zero-order beam B₂ (specular reflection) can be detected. Arelative angular position (φ) can be detected for each wavelength ofincident light, as follows: 2 dsin(φ)=mλ, wherein d is the period ofgrid, λ is the wavelength and m is integer corresponding to thediffraction order number.

It is thus evident that the present invention provides a simpleconstruction of an optical system having a deflector assembly with atleast one movable element capable of providing variable angles ofincidence and light collection with stationary mounted light source anddetector. The system can be easily adjusted for additionallyilluminating the same location (site) of the sample with a normalincident beam, as well as for imaging the same site.

Data indicative of the detected light is received at a control unithaving a suitable data processing utility, which is operable to analyzethe received data and determine at least one of the following sample'sparameters: reflectivity R as a function of wavelengths or angle ofincidence or both; diffraction efficiency as a function of wavelength atzero order diffraction; and typically ellipsometry measured parametersas Δ, which is a change in phase of the reflected beam from the incidentbeam, and ψ, which is defined as the arctangent of the amplitude ratioof the incident and reflected beams.

Those skilled in the art will readily appreciate that variousmodifications and changes can be applied to the embodiments of theinvention is hereinbefore exemplified without departing from its scopedefined in and by the appended claims

1. An optical system for use in measurements in a sample, the systemcomprising: (a) a light source operable to produce an incident lightbeam propagating in a certain direction towards the sample through anillumination channel; (b) a detector unit for collecting light comingfrom the sample through a detection channel, and generating dataindicative of the collected light; (c) a light directing assemblyoperable to direct the incident beam onto a certain location on thesample's plane with a plurality of incident angles, and to direct lightreturned from the illuminated location to the detector unit, the lightdirecting assembly comprising a plurality of beam deflector elements, atleast one of the deflector elements being movable, a position of said atleast one movable deflector element defining a selected one of theincident angles.
 2. The system according to claim 1l wherein theplurality of said deflector elements comprises two arrays of thedeflector elements, one array being located in the illumination channeland the other array being located in the detection channel.
 3. Thesystem according to claim 2, wherein each of the two arrays is formed bydeflector elements arranged in a spaced-apart relationship along therespective channel.
 4. The system according to claim 3, wherein thedeflector elements are mirrors having planar or parabolic-sectorreflecting surface.
 5. The system according to claim 4, wherein each ofthe deflector elements has its associated focusing lens.
 6. The systemaccording to claim 2, wherein each of the two arrays is formed by areflecting surface of a parabolic-sector mirror.
 7. The system accordingto claim 6, wherein the light directing assembly comprises a planarmirrors located in the optical part of the incident beam propagatingtowards the array of the deflector elements in the illumination channeland movable along this optical path to thereby reflect the incident beamonto the selected one of the deflector elements of the illuminationchannel.
 8. The system according to claim 7, wherein the light directingassembly comprises a second planar mirror accommodated in the detectionchannel in the optical part of the returned deflected beam and movablealong this channel, movement of the second mirror resulting in thedetection of the returned beam deflected by the selected one of thedeflector elements of the detection channel.
 9. The system according toclaim 1, wherein the plurality of the deflector elements comprises aparabolic-sector mirror facing the sample by its reflecting surface, andcomprises several planar mirrors operable together to direct theincident beam to the reflecting surface of the parabolic-sector mirrorto be reflected thereby onto said location on the sample, and to directthe returned beam to the detector unit, at least one of the planarmirrors being movable between a plurality of operative positions,thereby directing the incident beam onto a selected location on thereflecting surface and enabling obtaining a selected one of theplurality of incident angles.
 10. The system according to claim 1,wherein the detector unit comprises a spectroscopic detector.
 11. Thesystem according to claim 10, wherein the detector unit comprises apinhole accommodated upstream of the spectroscopic detector with respectto the direction of propagation of the returned beam towards thedetector unit.
 12. The system according to claim 11, wherein thedetector unit comprises an aperture stop accommodated upstream of thepinhole with respect to the direction of propagation of the returnedbeam towards the detector unit.
 13. The system according to claim 1,wherein the light directing assembly comprises a polarizing assembly.14. The system according to claim 13, wherein the polarizing assemblycomprises at least one polarizer located in the optical path of eitherthe incident light beam produced by the light source, or the returneddeflected beam propagating to the detector unit.
 15. The systemaccording to claim 1, wherein the light directing assembly defines anadditional illumination/detection channel for directing the incidentbeam onto the sample along an axis perpendicular to the sample's planeand directing a reflection of the perpendicular incident beam to thedetector.
 16. The system according to claims 6, wherein the lightdirecting assembly defines an additional illumination/detection channelfor directing the incident beam onto the sample along an axisperpendicular to the sample's plane and directing a reflection of theperpendicular incident beam to the detector, the additionalillumination/detection channel being formed by a beam splitter spatiallyseparating the perpendicular incident and reflected beams, and first andsecond planar mirrors, each being additionally shiftable between itsoperative and inoperative positions being, respectively, in and out ofthe optical path of the respective beam, such- that when the first andsecond planar mirrors are in their operative positions, the incident andreturned beams propagate through the illumination and the detectionchannels, respectively, and when the first and second planar mirrors arein their inoperative states, the incident and returned beams propagatethrough the additional illumination/detection channel.
 17. The systemaccording to claim 2, wherein the light directing assembly defines anadditional illumination/detection channel for directing the incidentbeam onto the sample along an axis perpendicular to the sample's planeand directing a reflection of the perpendicular incident beam to thedetector, the additional illumination/detection channel being formed bya beam splitter spatially separating the perpendicular incident andreflected beams, and first and second planar mirrors, the first mirrorbeing located in the optical part of the incident beam and beingshiftable between its operative and inoperative positions being,respectively, in and out of the optical path of the incident beam, andthe second planar mirror being accommodated in the optical part of thereturned beam and being shiftable between its operative and inoperativepositions being, respectively, in and out of the optical path of thereturned beam, such that when the first and second planar mirrors are intheir operative positions, the incident and returned beams propagatethrough the illumination and the detection channels, respectively, andwhen the first and second planar mirrors are in their inoperativestates, the incident and returned beams propagate through the additionalillumination/detection channel.
 18. The system according to claim 1,comprising an imaging channel defined by an imaging light source, animaging detector unit and an imaging lens assembly.
 19. (canceled) 20.An optical method for use in measuring in a sample, the methodcomprising: (i) providing an incident light beam propagating in acertain direction towards the sample along an illumination channel; (ii)directing the incident beam onto a certain location on the sample'splane with a plurality of incident-angles, said directing comprisingdeflecting the incident beam by a selected one of a plurality ofdeflector resulting in the selected one of the angles of incidence ofthe beam onto said certain location; and (iii) detecting light returnedfrom the illuminated location with a desired angle and generating dataindicative thereof to be analyzed for determining at least one parameterof the sample.
 21. The method according to claim 20, wherein said atleast one parameter includes at least one of the following: R(X),wherein R is the reflectivity of the sample and X is the wavelength ofincident light; R(6), wherein 8 is the angle of incidence; R(/%,9);diffraction efficiency as a function of wavelength at zero orderdiffraction ; a change A in phase of the returned beam from the incidentbeam; the amplitude ratio V of the incident and returned beams;A(X)and Y(X) for specularly reflected or zero order diffracted returned light.